Molarity Calculations in 5 Minutes for Non-Chemists

If you build lab management software, educational tools, or scientific calculators, you need molarity. The formula is simple but the unit conversions trip everyone up. The molarity formula M = n / V M = molarity (moles per liter, mol/L) n = number of moles of solute V = volume of solution in liters To find moles from mass: n = mass (g) / molar mass (g/mol) Combined: M = mass / (molar mass * volume) Practical example How much NaCl to make 500 mL of 0.9% saline (physiological saline)? 0.9% saline means 0.9 grams NaCl per 100 mL, or 9 g/L. The molar mass of NaCl is 58.44 g/mol. M = 9 / 58.44 = 0.154 mol/L For 500 mL: mass = M * molar_mass * volume mass = 0.154 * 58.44 * 0.5 = 4.5 g Dissolve 4.5 grams of NaCl in enough water to make 500 mL total volume. Dilution calculations The dilution equation: M1 * V1 = M2 * V2 M1 = initial molarity V1 = initial volume M2 = final molarity V2 = final volume To dilute 100 mL of 2M HCl to 0.5M: 2 * 100 = 0.5 * V2 V2 = 400 mL Add water to bring the volume